The present invention relates in general a method for forming non-woven webs and to the products produced thereby suitable for bath tissue or the like to heavier webs suitable for facial tissues, components for feminine napkins, diaper fillers, toweling, wipes, non-woven fabrics, saturating paper, paper webs, paperboard, et cetera.
Conventionally, materials suitable for use as disposable tissue and towel products have been formed on paper-making equipment by water-laying a wood pulp fibrous sheet. Following formation of the sheet, the water is removed either by thermal drying or by a combination of pressing and drying. As water is removed during formation, overall hydrogen bonding occurs at substantially all fiber intersections, and a thin, essentially planar sheet is formed. It is the hydrogen bonds between fibers which provide sheet strength, but due to this overall bonding phenomenon, cellulosic sheets prepared by water-laid methods inherently possess very unfavorable tactile properties (harshness, stiffness, low bulk, and poor overall softness) and poor absorbency characteristics.
To improve these unfavorable properties, water-laid sheets are typically creped from the dryer roll with a doctor blade. Creping reforms the flat sheet into a corrugated-like structure, thereby increasing its bulk and simultaneously breaking a significant portion of the fiber bonds, thus artificially improving the tactile and absorbency properties of the material. However, conventional creping is most effective on relatively low basis weight webs (less than about 15 lbs./2880 ft..sup.2), and higher basis weight webs, after creping, remain quite stiff and are generally unsatisfactory for uses such as quality facial tissues.
Sanford et al. U.S. Pat. No. 3,301,246 proposes improving the tactile properties of water-laid sheets by thermally predrying a sheet to a fiber consistency substantially in excess of that normally applied to the dryer surface of a paper machine and then imprinting the partially dried sheet with a knuckle pattern of an imprinting fabric. Creping of those areas knuckled to the dryer is still essential in order to realize the maximum advantage of the proposed process; and, for many uses, two plies are still necessary.
As will be apparent from the foregoing discussion, conventional paper-making methods have extreme water requirements which limit the locations where paper-making operations may be carried out. Such operations require removing a large quantity of the water used as the carrier, and the used process water can create an associated water pollution problem. Still further, the essential drying procedures consume tremendous amounts of energy.
Air forming of wood pulp fibrous webs has been carried out for many years; however, the resulting webs have been used for applications where either little strength is required, such as for absorbent products--i.e., pads--or applications where a certain minimum strength is required but the tactile and absorbency properties are unimportant--i.e., various specialty papers. U.S. Pat. No. 2,447,161 to Coghill, U.S. Pat. No. 2,810,940 to Mills, and British Pat. No. 1,088,991 illustrate various air-forming techniques for such applications.
In the late 1940's and early 1950's, work by James D'A. Clark resulted in the issuance of a series of patents directed to systems employing rotor blades mounted within a cylindrical fiber "disintegrating and dispersing chamber" wherein air-suspended fibers were fed to the chamber and discharged from the chamber through a screen onto a forming wire--see, J. D'A. Clark U.S. Pat. Nos. 2,748,429, 2,751,633 and 2,931,076. However, disintegration of the fibers by mechanical co-action of the rotor blades with the chamber wall and/or the screen mounted therein cause fibers to be "rolled and formed into balls or rice which resist separation"--a phenomenon more commonly referred to today as "pilling". These problems and proposed solutions thereto, are described in J. D'A. Clark U.S. Pat. No. 2,827,668, J. D'A. Clark et al. U.S. Pat. Nos. 2,714,749 and 2,720,005; Anderson U.S. Pat. No. 2,738,556; and, Anderson et al. U.S. Pat. No. 2,738,557.
A second type of system for forming air-laid webs of dry cellulosic fibers which has found limited commercial use has been developed by Karl Kristian Kobs Kroyer and his associates as a result of work performed in Denmark. Certain of these systems are described in: Kroyer U.S. Pat. Nos. 3,575,749 and 4,014,635; Rasmussen U.S. Pat. Nos. 3,581,706 and 3,669,778; Rasmussen et al. U.S. Pat. No. 3,769,115; Attwood et al. U.S. Pat. No. 3,976,412; Tapp U.S. Pat. No. 4,060,360; and, Hicklin et al. U.S. Pat. No. 4,074,393. In general, these systems employ a fiber sifting chamber or head having a planar sifting screen which is mounted over a forming wire. Fibers are fed into the sifting chamber where they are mechanically agitated by means of a plurality of mechanically driven rotors mounted for rotation about vertical axes. Each rotor has an array of symmetrical blades which rotate in close proximity to the surface of the sifting screen. The systems described in the aforesaid Kroyer and related patents generally employ two, three, or more side-by-side rotors mounted in suitable forming head.
This type of sifting equipment suffers from poor productivity because the rotor action concentrates most of the incoming material at the periphery of the blades where the velocity is at a maximum. Most of the sifting action is believed to take place in these peripheral zones, while other regions of the sifting screen are either covered with more slowly moving material or are bare, thereby producing a web with a non-uniform basis weight profile. Thus, a large percentage of the sifting screen area is poorly utilized and the system productivity is low. Moreover, fibers and agglomerates tend to remain in the forming head for extended periods of time, especially in the lower velocity, inner regions beneath the rotor blades. This accentuates the tendency of fibers to roll up into pills. Consequently, if the forming head is to be cleared of agglomerated material, it is necessary to remove 10% or more by weight of the incoming material from the forming head for subsequent reprocessing or for use in less critical end products. The separating method used (U.S. Pat. No. 4,014,635) entrains a large number of good fibers with the agglomerates leaving the forming head which are damaged by the hammermills in the secondary processing system.
The inventors have found that, when using high quality fibers in the Kroyer-type system, the above difficulties were aggravated. The rate of pill formation increased and it was necessary to remove and recycle more than 50% by weight of the incoming fibrous material to produce good quality tissue-weight webs. Productivity was unacceptably low and excessive damage was done to otherwise good fibers during the secondary hammermilling step. The tensile strength of the webs produced was decreased, and the circular movement of the rotors above the screen caused corresponding air and fiber movement in the forming region below the screen, resulting in basis weight nonuniformities.
In an effort to overcome the productivity problem, complex production systems have been devised utilizing multiple forming heads--for example, up to eight separate spaced forming heads associated with multiple hammermills and each employing two or three side-by-side rotors. The most recent sifting type systems employing on the order of eighteen, twenty or more rotors per forming head, still require up to three separate forming heads in order to operate at satisfactory production speeds--that is, the systems employ up to fifty-four to sixty, or more, separate rotors with all of the attendant complex drive systems, feed arrangements, recycling equipment and hammermill equipment.
During the 1970's a series of patents were issued to C. E. Dunning and his associates which have been assigned to the assignee of the present invention; such patents describing yet another approach to the formation of air-laid dry fiber webs. See: Dunning U.S. Pat. Nos. 3,692,622, 3,733,234 and 3,764,451; and, Dunning et al. U.S. Pat. Nos. 3,776,807 and 3,825,381. This development has been found to resolve a number of the problems that have heretofore plagued the industry. For example, high productivity rates have been achieved and fiber webs can easily be formed at high machine speeds. However, the system requires preparation of pre-formed rolls of fibers having high cross-directional uniformity and is not suitable for use with bulk or baled fibrous materials. Because of this, problems are experienced when attempting to scale the equipment up to produce wide webs--i.e., webs on the order of 120 inches in width or greater--and the requirement for pre-formed special web rolls having the requisite uniformity in cross-directional profile has been such that, to date, the system has found only limited commercial application.
Indeed, heretofore it has not been believed that air-forming techniques can be advantageously used in high speed production operations to prepare cellulosic sheet material that is sufficiently thin, and yet has adequate strength, together with softness and absorbency, to serve in applications such as bath tissues, facial tissues and light weight toweling.